The search for subsurface hydrocarbon deposits typically involves a multifaceted sequence of data acquisition, analysis, and interpretation procedures. The data acquisition phase involves use of an energy source to generate signals which propagate into the earth and reflect from various subsurface geologic structures. The reflected signals are recorded by a multitude of receivers on or near the surface of the earth, or in an overlying body of water. The received signals, which are often referred to as seismic traces, consist of amplitudes of elastic waves which vary as a function of time, receiver position, and source position. The data analyst uses these traces along with a geophysical model to develop an image of the geologic structure.
The analysis phase involves procedures which vary depending on the nature of the geological structure being investigated, and on the characteristics of the data set itself. In general, however, the purpose of a typical seismic data processing effort is to produce an image of the geologic structure from the recorded data. That image is developed using theoretical and empirical models of the manner in which the signals are transmitted into the earth, attenuated by the subsurface strata, and reflected from the geologic structures. The quality of the final product of the data processing sequence is heavily dependent on the accuracy of these analysis procedures.
The final phase is the interpretation of the processed results. Specifically, the interpreter's task is to assess the extent to which subsurface hydrocarbon deposits are present, thereby aiding such decisions as whether additional exploratory drilling is warranted or what an optimum hydrocarbon recovery scenario may be. In that assessment, the interpretation of the image involves a variety of different efforts. For example, the interpreter often studies the imaged results to obtain an understanding of the regional subsurface geology. This may involve marking main structural features, such as faults, synclines and anticlines. Thereafter, a preliminary contouring of horizons may be performed. A subsequent step of continuously tracking horizons across the various vertical sections, with correlations of the interpreted faults, may also occur. As is clearly understood in the art, the quality and accuracy of the results of the data analysis step of the seismic sequence has a significant impact on the accuracy and usefulness of the results of this interpretation phase.
Marine seismic data are occasionally recorded in regions characterized by moderate to highly irregular water-bottom surfaces or bathymetry. The Gippsland Basin offshore of Australia is an example. The seismic velocity contrast at the sea floor, in these regions, exhibits significant lateral variations which cause nonhyperbolic moveout in recorded seismic data. This can be more easily appreciated by realizing that the seismic velocity of water is roughly 1500 m/sec while that of sediment is on the order of 2200 m/sec. An irregular water bottom will, therefore, produce lateral seismic velocity changes on the order of 50% where the interface steeply dips. Commonly performed time-processing steps such as: normal move out (NMO), dip move out (DMO), stack and poststack migration; or prestack time migration; perform suboptimally when subjected to data contaminated with these distortions. This is because these processes assume reflection events always have hyperbolic form--an assumption which, due to the nonhyperbolic moveout, frequently is inaccurate. Therefore, the processing technician must either reduce the distortions prior to applying time-processing steps, or use relatively expensive prestack depth migration technology.
If the primary source of nonhyperbolic moveout in the data is sea floor related, then prestack depth migration would not be cost effective due to the large degree of irregularity introduced by the floor. This is exacerbated with three dimensional (3D) data, which is inherently much more expensive to manipulate than is two dimensional data, due to the added dimension. The most cost-effective approach in general is to reduce the nonhyperbolic distortion prior to the time-processing steps. However, for 3D seismic data, few satisfactory methods exist. Wave equation datuming is the theoretically preferred method; however, it is inapplicable to conventionally recorded 3D seismic. See, for example, Berryhill, "Submarine Canyons: Velocity Replacement by Wave-Equation Datuming Before Stack," Geophysics, Vol. 51, No. 8 (August 1986), pp. 1572-1579. Even if it were applicable, it would be prohibitively expensive. Static, corrections, another method known to those experienced in the art, see Sheriff, Encyclopedic Dictionary of Exploration Geophysics (3d ed. 1991) at p. 282, are adequate only if irregularity of the water bottom is small. Numerical ray tracing procedures, known as general replacement dynamics, are extremely expensive but precise. Sheriff, supra, at p. 242. Sometimes these procedures are used in a target-oriented way to make them affordable, but this compromises overall imaging. Another method has been proposed in Lynn, MacKay, and Beasley, "Efficient Migration Through Complex Water-Bottom Topography," Geophysics, Vol. 58, No. 3 (March 1993), pp. 393-398. The Lynn et al. method proposes substituting zero for the seismic velocity of, first, the sediment below the water bottom and, then, the water. However, it is not significantly more accurate than static correction.
Analogous difficulties arise in processing of land seismic data where there is a low seismic velocity layer (LVL) near the surface: again, there is a high seismic velocity contrast between the LVL and the formations below it. See copending U.S. patent application Ser. No. 08/134,808, now S.I.R. H001529 filed Oct. 12, 1993, which discloses a 2D solution for reducing distortions caused by the LVL.
There is therefore a need for a method to correct distortions in marine seismic data, or land seismic data over a LVL, particularly 3D marine or land seismic data, to reduce distortions caused by the seismic velocity contrast at the sea floor or the bottom of the LVL, which method is efficient and yields high quality images.